What do various property highlight color overlays mean?
Button colors reflect their respective property situation in relation to animation state.


Grey/Theme Colored - Unchanged, non animated value
Green - Property is animated but not keyframed at the current frame (notice the diamond outline decorator to the right)
Yellow - Property is animated and keyframed at the current frame (notice the filled diamond decorator to the right)
Orange - Value is changed but doesn't currently match the evaluated/keyframed animation for this frame. This happens when you changed the property but haven't yet keyframed the new value, it will jump back to the keyframed/evaluated value stored in the file if a new keyframed is not inserted, leading to potential data loss. The stronger orange color should warn the user of its unsaved state.
Blue - Property is overridden. This is only for libraries, or linked data from other external blend files. It means the value differs from the predefined default stored in the remote library, and has a new locally set data override (notice the overridden arrow decorator to the right).
Purple - Property has a Driver, an expression or simple math function that automatically controls its value according to input calculated from other properties/objects (notice the driven arrow decorator to the right).
Red - Error state (used more frequently in modifiers or constrains). The property either has an invalid value, is disabled or producing no visible effect. In the example below a Subdivision modifier has zero subdivisions both for viewport and rendered output and is thus effectively disabled for all intents and purposes. Despite no harmful errors occurring its name is shown red warning of its currently ineffective output.


Have in mind that all these colors are editable though, so while most themes try to stay close to these defaults, nothing stops them from using something completely different.